1. Field of the Invention
The present invention relates to a cardiac assist device and method, including an apparatus and method for detecting heart events such as for use in controlling the stimulation of a heart with a cardiac pacemaker.
2. Description of the Prior Art
An apparatus for detecting heart events is disclosed in U.S. Pat. No. 4,708,144 which includes a signal pick-up for acquiring an electrical signal dependent on the heart events, and a threshold detector following the signal pick-up for generating a detection signal indicating the presence of a heart event when the electrical signal exceeds a prescribed threshold. A signal evaluation means identifies the current signal height of the electrical signal exceeding the threshold with reference to the threshold. An averaging unit forms an average value from the threshold-related signal heights identified during a prescribed time interval. A control unit sets the threshold in relationship to the acquired, electrical signal dependent on the average value that has been formed, whereby the threshold is raised in relationship to the electrical signal given an increasing average value and is lowered given a decreasing average value.
The apparatus disclosed in U.S. Pat. No. 4,708,144 is a component of an implantable heart pacemaker and serves the purpose of detecting spontaneous heart events via the electrical signal of the intracardial electrogram, and thus controls the heart pacemaker. Given, for example, a single chamber heart pacemaker, it is thus possible to inhibit the stimulation of the heart given the appearance of detected, natural heart beats or--given a double chamber heart pacemaker--to synchronize the stimulation of one chamber with the heart events detected in the other chamber. A reliable detection of heart events requires a detection sensitivity that, on the one hand, is adequately high in order to be able to identify the signal component dependent on the heart activities from the acquired electrical signal, but is not so high that disturbances and noise signals (background components) are erroneously detected as heart events. Further, the selection of a defined detection sensitivity is made more difficult because the acquired electrical signal can change over a long term, for example because the electrodes employed for the derivation of the intracardial electrogram grow over after implantation over a longer time span in the heart.
An automatic matching of the detection sensitivity to the acquired electrical signal ensues in this known apparatus by using only heart events, among those which are detected, having an electrical signal which exceeds the aforementioned threshold; the threshold is set relative to the electrical signal by forming an average value from the signal heights of the electrical signals that exceed the threshold at every heart event during a time interval having a length in the range from at least a few minutes up to the order of magnitude of hours. The electrical signal is amplified with a variable gain dependent on the average value before it proceeds to the threshold detector. The threshold itself is set to an invariable value.
As disclosed in U.S. Pat. No. 4,827,934, however, it is also possible to modify the threshold itself for adapting the detection sensitivity to the electrical signal. Since the threshold is variable in this case, the detection margin, i.e. the respective spacing between the threshold and the signal height of the electrical signal exceeding the threshold, rather than the signal height itself, is utilized as parameter for setting the threshold. The conditions resulting in a variation of the threshold however, are not discussed in U.S. Pat. No. 4,827,934.
It is assumed in the adaptation of the detection sensitivity described in the aforementioned U.S. Pat. No. 4,708,144 that the signal parts of the acquired electrical signal which correlate with the heart events to be detected are exhibit only extremely slow change, for which reason the formation of the average value that is undertaken ensues over several minutes up to several hours. Although this known device is successful in nearly completely eliminating the influence of noise signals on the setting of the detection sensitivity, the adaptation of the detection sensitivity to, for example, a rapidly steady or a suddenly discontinuous variation of the electrical signal ensues corresponding slowly since such a change--due to the long time interval for averaging the signal height of the electrical signal--arises only relatively late in the average value that serves as the parameter for changing the detection sensitivity. Such sudden or rapid changes of the electrical signal can be based on suddenly occurring disturbing influences such as noise signals from the outside, changes in the electrode position, or sudden changes of the heart signals itself, for example after extra heartbeats or given an infarction, and can therefore cannot be precluded.